1. Field of the Invention
The present invention relates to a scanning image forming lens and an optical scanning apparatus.
2. Description of the Related Art
An optical scanning apparatus for optically scanning a surface to be scanned at a constant velocity by deflecting a luminous flux emitted from a light source via an optical deflector so as to transmit the luminous flux through a scanning image forming lens to be condensed into an optical beam spot on the surface to the scanned is well known in an image forming apparatus such as a laser printer, a digital copier, a facsimile machine and other such devices. The luminous flux is deflected in a direction corresponding to a main scanning direction for the surface to be scanned at equiangular velocity by rotation of the optical deflector such that the optical beam spot formed by the scanning image forming lens scans the surface to be scanned at a constant velocity in the main scanning direction. Typically, the surface to be scanned includes a photoconductor and the optical beam spot forms picture elements which constitute an image to be formed on the surface to be scanned.
The above-mentioned main scanning direction refers to the direction corresponding to the main scanning direction for a surface to be scanned along a light path from a light source to the surface to be scanned. A direction corresponding to a sub scanning direction refers to the direction corresponding to a sub scanning direction for the surface to be scanned along the light path. The sub scanning direction is substantially perpendicular to the direction corresponding to the main scanning direction.
In such an optical scanning apparatus, when the optical beam spot scans the surface to be scanned in the main scanning direction, if the diameter of the optical beam spot changes depending upon the position of the optical beam spot in the main scanning direction on the surface to be scanned, the size of each picture element, which is formed by the optical beam spot on the surface to be scanned so as to form an image on the surface to be scanned, changes depending upon the position where each picture element is written on the surface to be scanned in the main scanning direction. Consequently, the resolution of the formed image changes along the main scanning direction, resulting in deteriorating of image quality. A position on the surface to be scanned in the main scanning direction is sometimes referred to herein as "image height".
A change in he diameter of the optical beam spot in the main scanning direction on the surface to be scanned according to the position of the optical beam spot in the main scanning direction can be corrected to a certain degree, for example, by adjusting the time for writing each picture element on the surface to be scanned with the optical beam spot. However, a change in the diameter of the optical beam spot in the sub scanning direction according to the position of the optical beam spot in the main scanning direction cannot be corrected by adjusting the writing time for the optical beam spot. The change in the diameter of the optical beam spot in the sub scanning direction according to the position of the optical beam spot in the main scanning direction is typically corrected through adjustment of the optical performance of an optical lens system used for transmitting the light flux, which is deflected by the optical deflector in the direction corresponding to the main scanning direction, such that the luminous flux is condensed into an optical beam spot on the surface to be scanned. The optical lens system for transmitting the deflected light flux so as to form an optical beam spot and to scan the surface to be scanned with the optical beam spot is herein called a scanning image forming lens.
It is known that a change of an optical beam spot diameter in the sub scanning direction on a surface to be scanned according to the image height can be suppressed by correcting the curvature of field of the scanning image forming lens in the sub scanning direction. Typically, correction of the curvature of field in the sub scanning direction is made only relative to a paraxial luminous flux of the scanning image forming lens system.
However, correction of the curvature of field of the scanning image forming lens must be performed while keeping the other optical characteristics of the scanning image forming lens at a satisfactory level. For example, the constant velocity characteristics must be kept at a satisfactory level for enabling the optical beam spot to be moved at a constant velocity along the surface to be scanned. Typically, when the curvature of field is corrected, the other optical characteristics deteriorate.
In addition, when, for example, a photoconductor is used in an optical scanning apparatus as the surface to be scanned for forming an image thereupon, an assembly tolerance of the photoconductor in relation to the scanning image forming lens must be carefully considered in the design of the apparatus for accomplishing a desired level of the curvature of field in the scanning image forming lens. More specifically, even when the curvature of field of the scanning image forming lens is accurately corrected in the design of the scanning image forming lens, the curvature of field of the scanning image forming lens as designed may not necessarily be realized if the actual position of the surface to be scanned in relation to the scanning image forming lens is deviated from the designed position due to, for example, an assembling error relating to the position of the surface to be scanned relative to the scanning image forming lens.